1. Technical Field
This invention relates generally to oil well instrumentation and controls and more particularly to communication networks in well monitoring and control systems implemented over a communications bus.
2. Related Background Art
Permanent monitoring systems, as shown in FIG. 1, are used to equip subterranean wells with downhole sensors and devices, enabling the well operator to monitor and control downhole conditions in a completed well.
Generally, permanent monitoring systems are found in oil or gas wells, but can also be used in water or geothermal wells. After drilling is complete, the borehole is typically cased with casing 2 and cemented (cement annulus 3) along its entire length in formation 4, and closed by wellhead 5. Typically, the casing 2 has perforations P at an oil producing zone 6 to allow the oil to enter the cased well for production Well xe2x80x9ccompletionxe2x80x9d includes the installation within the cased well 1 (With direction of production flow indicated by the arrow) of production tubulars (or tubing) 18 and downhole devices including packers 10, valves 11, other devices 12 and sensors 25 necessary to bring the production safely and efficiently to surface. Completion equipment typically remains installed in the well for the duration of production.
Downhole sensor 25 for measuring, for example pressure, temperature, flow, resistivity or other physical parameters, is commonly attached to the tubing 18 at selected locations.
Advanced permanent monitoring and control systems, especially useful where several zones traversed by the well are producing (as occurs, for example, in horizontal or multilateral wells) typically employ downhole sensors and actuators, controllable from the surface, to improve production efficiency of the well.
In these installations, downhole devices including, for example (with reference to FIG. 1), packer 10, valve 11, other device 12 and sensor 25, are electrically connected through cable 20 to a data acquisition device 15 usually located at the surface. The cable is permanently installed along with the tubing during well completion.
Permanent cable 20 is secured by clamps 30 to the tubing 18 and lowered into the well with the tubing. Cable 20 is used both to provide necessary electrical power to the sensors and to carry the measurement information as an electrical signal, in analog or digital form, to the data acquisition device at the surface.
In present permanent monitoring systems, permanent cable 20 is typically implemented as an encapsulated mono-wire cable 20 consisting of insulated copper mono-wire 22 housed within a stranded (conductive) armor 24. This configuration, however, because of the harsh downhole environment, has proven difficult to maintain in an efficiently operable state for long periods of time due to degradation, the biggest problem being loss of the cable""s watertight characteristics.
As illustrated in FIG. 2, conductive metal clad cable 20xe2x80x2, comprising the insulated copper mono-conductor 22 housed in conductive metal clad armor 24xe2x80x2, is now preferred to stranded armor cable for its advantages in maintaining watertight operation for longer periods of time.
The mono-wire 22 of metal clad cable 24xe2x80x2 currently in use is typically an AWG#18, 7 strand copper mono-conductor with double layer insulation, having a DC resistance of 23 xcexa9/km. The metal clad armor 24xe2x80x2 is typically stainless steel tubing having an outside diameter of 6.35 mm and a DC resistance of 9 xcexa9/km.
A typical application of such cable is illustrated in FIG. 3 where, as cable 20xe2x80x2 is mono-conductor, the electrical circuit between a downhole device such as sensor 25 and the surface device 15 is closed by the (conductive) metal clad armor 24xe2x80x2 of the cable.
Limitations of mono-conductor cable in permanent cable applications restrict the effectiveness and utility of the current level of advancement achieved in the area of permanent monitoring systems.
For example, mono-conductor cable is not especially well adapted to use as a transmission line as the electrical parameters are not well defined and are dependent on the tubing, limiting the useful frequency bandwidth and data rate of transmission.
The mono-conductor cable can only support DC or AC power from surface to downhole to supply downhole sensors with power, and communication signals, one-way or two-way, between surface and downhole. These signals are not xe2x80x9cfloatingxe2x80x9d as they are tied to ground by the tube and tubing return path. No additional power can be sent, for example, to operate downhole actuators.
The armor (and tubing) return path in the mono-conductor cable can also be used by currents from other devices, like another sensor and its cable installed in the same well or a downhole electric motor powered from the surface through a power cable to operate a downhole pump. In such cases, unwanted signals will likely be added to the system signals and will appear as noise. Also, as the conductor and the return path are physically separated, electromagnetic signals could be induced in this circuit loop, increasing the noise level of the communication link and disturbing the wanted signal readability.
The advantages of twisted pair lines in lieu of single conductor lines in traditional communications applications are well known and have been applied to some degree in the petroleum industry.
For example, U.S. Pat. No. 5,444,184 to Hassel utilizes at least two twisted pairs for above surface communications between land and offshore installations. Hassel discusses the addition of signal communication paths to a power transmission cable by replacing each power conductor with a twisted pair wire. In one embodiment, 3 signal communication paths are added to a 3 phase power cable by replacing each phase conductor by a twisted pair. In another embodiment, 2 signal communication paths are added to a 2 conductor monophase power cable by replacing each conductor by a twisted pair.
The cable of Hassel is not proposed for downhole installation inside a well, but for placement on the sea floor to link the shore and the well head. Differential connections are utilized for both communication and power transmission. Hassel is neither intended as a subsurface transmission line (i.e., for downhole well applications) and exploits the long known advantageous characteristics of utilizing twisted pairs lines in communications applications.
Another example of the application of twisted pair lines in petroleum industry applications is found in U.S. Pat. No. 4,646,083 to Woods which discusses use of a twisted pair line to transmit DC power and analog signals (frequency) in downhole applications. DC power and two AC communication signals are multiplexed on the 2 wires of a twisted pair using a differential mode (i.e., the current is sent via the first wire and returns via the second). The multiplexing element is a transformer associated with a DC current balance circuit to avoid saturating the magnetic core of the transformer buy the transmitted DC power current.
Limitations of the prior art are overcome by the method and apparatus of the present invention of an oil well monitoring and control system communication network as described herein.
In various embodiments of the apparatus and method of the present invention for transmission of electrical signals, including both electrical power and communication signals, between a plurality of locations in a well, a bus supervisor is located at one of the plurality of locations with a node located at each of the remaining locations of the plurality of locations.
A bus interface at the bus supervisor and each node is capable of supporting differential and common mode connection between the bus supervisor and each node, and between the nodes themselves. The interface provides high parallel impedance for differential mode connections and low series impedance for common mode connections.
A bus electrically connects the bus supervisor and each node via the respective bus interfaces in common mode with an electrical return path to the bus supervisor for transmitting electrical power signals to or between each node location, and in differential mode for two way transmission of communication signals between the bus supervisor and nodes or between the nodes.
In alternative exemplary embodiments, the bus interface can be coupled to either a differential receiver and a coil, a differential transmitter and a coil, or it can provide galvanic insulative properties such as when implemented as a transformer.
The bus can comprise a cable comprising a pair of electrical conductors and an external armored shell housing the conductor pair. The armored shell can also be conductive and be used as the common mode electrical return path.
In a preferred embodiment, the bus is implemented as a twisted pair cable.
Communication, including data and control, signals and power, including both AC and DC, signals power can be transmitted. The bus supervisor may be located at either a surface or downhole location.
The present invention can also be adapted to multi-node and industrial bus extension applications.
Advances in the art are achieved by accommodating both power and communication applications where digital signal transmission is accomplished in differential mode, and DC or AC power transmission in common mode with return, in a preferred embodiment, via a twisted pair cable communication bus. The transformers in the various embodiments are not subject to saturation as in known implementations as the power current is automatically balanced into the two transformer primary windings. No current balancing circuit is necessary and a single twisted pair suffices for both power and communication signal transmission.
The foregoing and other features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawings.